Network-Controlled Repeater Type 2-O

Description

NCR type 2-O is a network-controlled repeater (NCR) specifically designed for operation in Frequency Range 2 (FR2, typically 24.25 GHz to 71 GHz), which encompasses millimeter wave (mmWave) bands. Like other NCRs, it consists of two logical entities: the NCR Mobile Termination (NCR-MT) and the NCR Forwarding (NCR-Fwd). The NCR-MT maintains a control link with the network (e.g., a gNB) to receive configuration and control information, while the NCR-Fwd amplifies and forwards the user data signals between the gNB and UEs. In type 2-O, both entities operate at FR2, meaning the control link and the forwarded signals are in the mmWave range.

The key characteristic of type 2-O is that its requirement set consists only of over-the-air (OTA) requirements defined at the Radiated Interface Boundary (RIB). This means that all performance testing and validation are done through radiated measurements, without any conducted test points. This is a practical necessity for FR2 because at mmWave frequencies, antenna arrays are typically integrated with the radio, and there are no standardized conducted connectors; the only feasible way to test is over-the-air. The RIB is a virtual boundary around the NCR equipment where radiated emissions and sensitivity are measured. OTA requirements at the RIB ensure that the NCR meets performance criteria such as output power, error vector magnitude (EVM), and blocking characteristics when tested in a radiated setup.

Type 2-O is intended for deployment in mmWave bands to extend coverage and fill coverage holes, which are more prevalent due to higher path loss and blockage. The NCR-MT allows the network to control the forwarding direction, gain, and beamforming, similar to type 1-H but adapted for FR2. Because of the high frequency, beamforming is essential, and the NCR-Fwd uses phased arrays to create narrow beams that can be steered electronically. The network can adjust these beams based on UE location and traffic demands. The OTA-only requirement set simplifies the equipment design because it eliminates the need for conducted test ports, but it also means that testing must be done in an anechoic chamber or compact antenna test range, which can be more complex and expensive.

In the overall network architecture, NCR type 2-O acts as a relay-like node but without the need for a full backhaul link; it simply amplifies and forwards signals. It can be deployed on lampposts, building walls, or indoor ceilings to improve mmWave coverage. Because it is network-controlled, it can coordinate with the gNB to avoid interference and optimize beam selection. The NCR-MT uses the same NR Uu interface for control as a UE, but with specific adaptations for NCR. The NCR-Fwd operates in the same frequency band as the access link, potentially causing self-interference, which is managed through isolation techniques and network scheduling.

3GPP introduced NCR type 2-O in Release 19, with specifications in TS 38.106 and TS 38.114. In Release 20, the wording of the definition was changed to state that the NCR includes NCR-MT or NCR-Fwd operating at FR2, rather than both, clarifying that not both entities must be present in all implementations. However, the fundamental OTA-only requirement set at RIB remains. This evolution reflects a more flexible approach to NCR design, allowing for scenarios where only the forwarding part is active or only the control part is present, depending on the deployment needs. Type 2-O is expected to play a crucial role in 5G-Advanced and future networks by enabling cost-effective mmWave coverage extension.

Purpose & Motivation

NCR type 2-O was created to address the specific challenges of coverage in FR2 (mmWave) bands, where signal propagation is severely limited by high path loss, atmospheric absorption, and blockage. Traditional repeaters are not suitable for mmWave due to their lack of beamforming and network control, which can lead to interference and inefficient use of spectrum. NCR type 2-O provides a network-controlled amplification solution that can be integrated into the network's beam management and interference coordination frameworks. The decision to use only OTA requirements at RIB stems from the impracticality of conducted testing at mmWave frequencies, where antenna arrays are integrated and no standard connectors exist. By defining only OTA requirements, 3GPP ensures that performance can be validated in a realistic radiated environment. The introduction of NCR type 2-O in Release 19 was motivated by the need for economical coverage extension in mmWave deployments, particularly for indoor and outdoor hotspots, without the need for fiber backhaul. The Release 20 wording change allows for more flexible implementations, recognizing that some NCRs may only need to implement the forwarding function or the control function, depending on the use case.